User:Tim Zukas/Lat-lon

Lat-lon: what are they, anyway?

Draft: lat-lon
If you had asked NGS for the location of the Empire State Building in Manhattan a few decades ago they would have told you


 * Latitude 40 deg 44 min 53.984 sec NorthLongitude 73 deg 59 min 09.842 sec West

Whereas now they would tell you


 * Lat 40-44-54.36051 NLon 73-59-08.35715 W

If you asked about the Transamerica Pyramid in San Francisco, a few decades ago they would have said


 * Lat 37-47-42.82630 NLon 122-24-06.07934 W

But now they would say


 * Lat 37-47-42.57543 NLon 122-24-09.97865 W

So what's going on? Has either building moved that much? Was the earlier position wrong? Or how about the tall water tank a few hundred meters north of the Hilo airport, on the Big Island of Hawaii, which used to be


 * Lat 19-43-54.526 NLon 155-03-26.463 W

but is now


 * Lat 19-43-43.61544 NLon 155-03-16.47749 W

Has it moved 300 meters to the southeast since, say, 1980? Or was the old lat-lon that wrong?

Probably neither; the problem is that "latitude" and "longitude" aren't as absolute as we thought, even if all the measurements are as perfect as they can possibly be.

(By the way: the UTM Northing for the Empire State Building was 4511102.757 meters in the old system; in the new it's 4511326.208. And that's the last you'll read about UTM in this article.)

Since the earth isn't actually spherical, we might have some trouble spelling out exactly what we mean by "latitude", but "longitude" seems simple enough: if it takes 23 hours 56 minutes for the earth to make a rotation, relative to distant stars, then when we're at exactly 120 degrees west longitude a star should pass overhead 7 hours 58 min 40 sec after it passed over Greenwich. But it turns out it isn't so easy to define "overhead"; we have to use a plumb bob or equivalent device, and we can't expect the plumb bob to always point to the center of the earth-- if a mountain is west of us and a deep ocean is east of us, the mountain will attract the plumb bob enough to throw our "overhead" off. Tim Zukas (talk) 21:15, 23 August 2010 (UTC)